Last week it was the price of cabbages. This week it's Treaty rights and the price of race relations, writes Vernon Small.

It's amazing how the debate sparked by Labour's move to charge royalties on water has gone from the ridiculous to the constitutionally sublime.

And on the way National has pivoted towards Winston Peters' voters as it tries to shore up support in the face of the Jacinda Effect.

Since Thursday National and its farming allies, as well as Peters, cashed in on Labour's refusal to put a definite figure on what it would charge farmers for irrigation water.

Into the void were poured some crazy numbers including claims it would push up the price of cabbages to $18, heap billions onto farming costs and even put a cheeky Chardonnay out of reach of the average quaffer.

It was all nuts, but it flushed Labour out. Its water spokesman David Parker on Sunday confirmed the royalty would be 2c/1000 litres tops on irrigation water.

That should have calmed the farm.

With 5 trillion litres of irrigation water allocated across the economy the total impost would be $100m. International studies suggest a litre of milk and a litre of wine need about 1000 litres of water to produce. So that would add 2c to the cost of producing a litre of milk and just 1.3c to that bottle of wine.

A cabbage would cost just 0.6c more to produce.

And those numbers all assume the total water required would come from irrigation - and its worth remembering it does rain in Aotearoa

By Monday Prime Minister Bill English was still labelling Labour as untrustworthy but seemed to accept the royalty would be low. "So why bother?"

But he and Treaty Negotiations Minister Chris Finlayson have radically changed tack.

Labour had, they said, blundered into a major concession with constitutional and Treaty implications. By imposing a water royalty they have asserted Crown ownership and that opened the way to iwi claims too.

It is hard to see, in that case, why National is even examining the issue of charging for bottled water, since presumably that would open the same Pandora's box.

But the thrust was all very "Seabed and Foreshore" – though lite.

English said Labour's move would undermine years of careful negotiations that had kept ownership at bay while they dealt with the rights and interests of Maori and moved towards "allocation" of water.

Finlayson even raised the prospect of "full and final" Treaty settlements being reopened over water.

Parker rubbished both ideas.

He said charging a royalty no more asserted ownership than charging GST meant the Government owned all fruit and veg (yes, including cabbages).

As far as settlements are concerned, he said they already contained clauses that acknowledged fresh water claims were unresolved.

It seems likely Labour is prepared to consider possibly a "Water-lords" equivalent of the pan-Maori Sealords deal that gave iwi a share of the fishery – something English has ruled out.

Peters meanwhile can hear the strains of one of his top ten election hits.

"This is as serious as it gets. The two old parties has National selling out and writing race-based water ownership into law whilst accusing Labour's proposed water tax of being the trigger to justify it."

It all seemed so much simpler when we were only arguing about the price of a cabbage.